Sensors for measuring the mass flow rate which operate in accordance with the Coriolis principle are already known in various designs from the prior art, these generally being embodied in a conventional design, that is to say not in a micromechanical design. U.S. Pat. No. 4,768,385 discloses such a sensor, for example, two line sections being provided which extend in two planes running parallel to one another. These two line sections can be excited to effect oscillations in antiphase, an electrical measuring device for measuring the oscillation behavior being provided at the location of the greatest deflection. The measurement principle is based on the so-called Coriolis principle. This exploits the physical effect that fluids which are flowing, that is to say which are in motion, assuming that they are conducted through a rotating or oscillating fluid conductor, generate Coriolis forces that act at right angles to the flow direction of the fluid, on the one hand, and to the direction of movement of the fluid conductor, on the other hand. The magnitude of said Coriolis forces is proportional to the product of the mass flow and the angular velocity of the fluid. By measuring the Coriolis force, with a known geometry of the fluid conductor it is thus possible to calculate the mass flow of the fluid.
DE 34 43 234 A1 describes a mass flow rate sensor in accordance with the Coriolis principle which has a different design. In this sensor, the line sections used for measurement have a rectilinear course and are arranged parallel to one another. In order to carry out the measurement, the line sections are caused to effect bending oscillations.
Enoksson et al. in “A Silicon Resonant sensor structure for Coriolis mass flow managements”, Journal of Micro electro Mechanical Systems, Vol. 6, No. 2, June 1997, make a proposal as to how mass flow sensors in accordance with the Coriolis principle can be embodied in a micromechanical design. The micromechanically produced sensor is produced from two bonded silicon wafers into which half-shells of the line sections to be produced have respectively being produced. The half-shells are completed to form a closed line cross section. Owing to the outlay associated with the anisotropic etching of silicon wafers, the mass flow rate sensor is produced only in two layers. For this purpose, two line sections are produced which lie in one plane in accordance with the geometrical extent of the wafers. The two line sections can be excited to effect oscillations in different oscillation modes, it being apparent that different sensor quality factors can be achieved depending on the chosen type of excitation. The sensor quality factor (Q) is a measure of the damping of the oscillatory system which directly influences the sensitivity of the relevant sensor structure. Quality factors of Q<=1500 can be achieved with the mass flow rate sensors of micromechanical design as proposed by Enoksson.